ADO - how to get primary key fields - database

Question

I'm using ADO (Delphi & C++ Builder) and I would like get primary key fields (their names) that are in some TADOTable component. How to do it?
I found out that I need to use ADOConnection->OpenSchema but don't know how to use this..
Now I tried this:
int bounds[] = {0,2};
OleVariant A(bounds,1, varVariant);
A.PutElement(varEmpty,0);
A.PutElement(varEmpty,1);
A.PutElement("MyDBTable",2);
OleVariant EmptyParam;
EmptyParam.VType = VT_ERROR;
EmptyParam.VError = DISP_E_PARAMNOTFOUND;
TADODataSet *temp = new TADODataSet(NULL);
AdoConnection1->OpenSchema(siPrimaryKeys, A, EmptyParam, temp);
temp->Open();
temp->First();
while (!temp->Eof)
{
Memo1->Lines->Add(temp->Fields->Fields[0]->AsString);
temp->Next();
}
temp->Close();
delete temp;
When running this code I get: "Object or provider is not capable of performing requested operation."?

Answer 1

References for OpenSchema Method (ADO), examples can be found here
An example implemention in Delphi for Microsoft Access and MSSqlServer could look like this:
Procedure OpenPrimaryKeyInfo ( Connection:TAdoConnection
; DatabaseName , SchemaName , TableName : Variant
; Display:TAdodataset );
begin
Connection.OpenSchema( siPrimaryKeys
, VarArrayOf([ DatabaseName , SchemaName , TableName ])
, EmptyParam , Display );
end;
Example call for Microsoft Access:
OpenPrimaryKeyInfo( AdoConnection2 , UnAssigned , UnAssigned , 'TableX' , Adodataset1 );
Example call for MSSqlServer:
OpenPrimaryKeyInfo( AdoConnection1 , 'MyDataBase' , 'dbo' , 'TableX' , Adodataset1 );

Answer 2

You establish the connection and open it as usual (using TADOConnection.ConnectionString and TADOConnection.Open), and then ask for the schema using OpenSchema. The TADODataSet you provide as the last parameter will contain a RecordSet that you can use just like any other dataset.
Here's a quick sample I threw together (thanks to #bummi for the correction to the third parameter - Unassigned and Null both compiled, but didn't actually work when tested). I dropped a TADOConnection, TADODataSet, and TMemo on a new form and configured the TADOConnection quickly to point to a simple SQL Server Express database I have for some testing - I included the connection string; the only change I made to it was in the computer name provided in the Data Source portion).
procedure TForm3.FormShow(Sender: TObject);
var
i: Integer;
sLine: string;
begin
Memo1.Clear;
ADOConnection1.ConnectionString := 'Provider=SQLOLEDB.1;' +
'Integrated Security=SSPI;' +
'Persist Security Info=False;' +
'Initial Catalog=Contacts;' +
'Data Source=MyComputer\SQLEXPRESS';
ADOConnection1.Connected := True;
ADOConnection1.OpenSchema(siPrimaryKeys, Unassigned, EmptyParam, ADODataSet1);
sLine := '';
for i := 0 to ADODataSet1.FieldCount - 1 do
sLine := sLine + ADODataSet1.Fields[i].FieldName + #9;
Memo1.Lines.Add(sLine);
Memo1.Lines.Add('');
while not ADODataSet1.Eof do
begin
sLine := '';
for i := 0 to ADODataSet1.FieldCount - 1 do
sLine := sLine + ADODataSet1.Fields[i].AsString + #9;
Memo1.Lines.Add(sLine);
ADODataSet1.Next;
end;
end;
The possible values for the SchemaInfo value (the first parameter passed to OpenSchema) can be found in the ADODB unit - they're documented in the Delphi help file (note that the documentation says that not all of them are available via ADO):
TSchemaInfo = (siAsserts, siCatalogs, siCharacterSets, siCollations,
siColumns, siCheckConstraints, siConstraintColumnUsage,
siConstraintTableUsage, siKeyColumnUsage, siReferentialConstraints,
siTableConstraints, siColumnsDomainUsage, siIndexes, siColumnPrivileges,
siTablePrivileges, siUsagePrivileges, siProcedures, siSchemata,
siSQLLanguages, siStatistics, siTables, siTranslations, siProviderTypes,
siViews, siViewColumnUsage, siViewTableUsage, siProcedureParameters,
siForeignKeys, siPrimaryKeys, siProcedureColumns, siDBInfoKeywords,
siDBInfoLiterals, siCubes, siDimensions, siHierarchies, siLevels,
siMeasures, siProperties, siMembers, siProviderSpecific);